I heard it can be done. Anyone know how? Is there another cheap way to do it. I just want to measure the idle rpm. Thanks.

 

Well, there's probably many ways to do it, but the first that comes to mind is, if your meter is capable - to measure the number of ground pulses in a given amount of time on the lead running from the CDI to the coil, then do the appropriate math.

Another way, if you had a inductive clamp style meter, would be to clamp the spark plug wire, measure the number of voltage spikes over a given amount of time, then do the appropriate math.

 

To figure out the frequency that the Fluke gives you,(if it will even read it), take your number and multiply it by 60 and you should have the RPM's.

An example would be if it were running at 750 RPM's, you should get about 12.5 Hz(750/60=12.5).

Just a thought, but what kind of voltage and current would be comming out of the coil? (1000's of volts?) Most meters only go up to 1 or 1.5 kv.
ERIC

 

You just need to see the pulses, not know the actual voltage.

You'd also want to multiply the final number by 2 since we're dealing with a 4 stroke and it only fires once for every 2 360 degree revolutions(unless it's a wasted spark system).

There are also inductive tachometers that will measure both RPM and coil voltage available that aren't too awfully expensive. We've got one and that's what I use.

 

the better fluke meters (at least the automotive variety) come with an inductive RPM pickup, it just clamps around the plug wire... a pretty pricey proposition just to measure idle rpm..

a cheaper way? go to your local small engine repair shop and pick up a mechanical tach.. ~$5, they look stupid, and take some getting used to, but work quite well.

I'm pretty sure the 400EX is a waste spark system (plug fires every rotation) so be sure to figure that into the math..

Leo

 

I was also thinking it was a wasted spark setup. I believe I had to set my inductive tach to "2 stroke" to get an accurate reading but couldn't remember for sure....

 

You can use something small engine mechanics use, I call it a wibblemeter and you hold it on the motor, it's pretty difficult to explain but simple in operation. They cost around $20. Does anyone here know what I am talking about? Maybe you can give a good explanation.

 

Gabe,
I might be mistaken, but doesn't the plug spark on every tdc? Even tho it is not a compression stroke?

OOPS, Sorry, I should have continued reading....

 

sorry ,i was sleepin

 

huh?

1 complete revolution of the crankshaft in one minute = 1 rpm (rev/minute)

the camshaft spins at 1/2 engine speed..

Leo